1. Field of the Invention
The present invention relates to microgyros and their methods of manufacture, particularly the structure and manufacturing methods for mechanical resonators in micro- and meso-gyro scopes.
2. Description of the Related Art
Miniature high grade mechanical resonators have a wide range of applications including micro- or meso-scale gyroscopes. Tuning is a critical portion of the manufacturing process of high grade mechanical resonators, particularly when the size of the resonator approaches a meso or micro scale. The rough manufacturing process of the resonator does not obtain acceptable precision. Generally, such tuning involves a selective 3D removal of material from the resonator in order to alter its physical properties and effect a change in the resonator behavior, e.g. a change in the natural frequency. This fine adjustment is used to obtain a very precise resonant frequency and/or a high Q balanced vibration of the resonator. Precision in the material removal process is essential.
For example, micromachining of axisymmetric silicon resonators for gyroscopes using deep reactive ion etching (DRIE) leaves a fixed residual resonant frequency mismatch of 0.1 to 1 Hz for mesoscale resonators and 10 to 30 Hz for mm scale microgyro resonators. One explanation for the limited results is that a fixed photo-mask is used in this process that affords no adjustment of the resonator in-situ. Furthermore, this process is also limited because it is very difficult to apply to an assembled gyroscope.
Gyroscope performance depends, in part, upon the frequency mismatch of the resonator. For example, gyroscope quadrature drift is proportional to the frequency mismatch of the resonator. Because navigation grade gyroscopes require high-Q resonators with milli- or micro-Hz matching of kHz resonances, the raw results of such DRIE micromachining processes (producing frequency mismatch over 1 Hz) is unacceptable. Additional processes to tune the resonator are necessary.
Tuning with laser assisted gas etching or laser ablation after the micromachining process has been used, but the precision of these processes does not yet produce acceptable results for many applications. In addition, gas reaction products or debris liberated during these processes may damage the resonator or assembled gyroscope. Furthermore, gases may also damp the resonance, distorting any-in-situ measurement. Although tuning precision is critical, the selected tuning process must also be compatible with the materials and structure of the resonator.
For some resonators, such as capacitive gyroscopes, electrostatic bias trimming can be used to induce an unbalanced electrostatic spring softening to adjust one or more resonance frequencies. However, this technique is subject to electrical errors and thermal tracking of mechanical and electrical errors.
There is a need for methods of manufacturing mechanical resonators with very high mechanical precision. There is further a need in the art for such manufactured mechanical resonators in applications such as microgyros with greatly improved performance for navigation. The present invention meets these needs.
It should be noted that throughout this specification, the invention will be described in relation to meso- or microgyroscopes. Although the invention may be more desirable for smaller gyros (where even small changes in the resonator have a greater performance impact), the size scale of the resonator does not determine the applicability. The invention can be applied to any mechanical resonator requiring high precision, based upon the desired gyro performance.
Embodiments of the invention perform a novel xe2x80x9cnanomachiningxe2x80x9d method for tuning mechanical resonators that includes other beneficial properties over related tuning processes. An exemplary method includes mounting a mechanical resonator with means for exciting and sensing its resonant frequency in a vacuum chamber along with a focused ion beam. The resonant frequency is then adjusted to a desired resonant frequency value using the focused ion beam to remove very small amounts of material at a sufficiently slow rate to controllably change the dimension of the resonator at a sensitive location, e.g. the resonator flexure beam thickness.
Such a method has application in producing navigation grade vibratory gyroscopes achieving below parts-per-million tuning precision of their resonance frequencies. Furthermore, because the focused ion beam process is benign to the resonator materials and structure, the built-in narrow gap electrostatic actuators and sensors of the gyroscope can be used in the tuning process.
Selection of the appropriate sensitive locations for material removal (and approximate amount of material to remove) can be determined by a finite element model analysis of the normal modes of the mechanical resonator. The finite element model analysis should have sufficient resolution to represent the structural modes before and after a proposed quantity of material is removed.
Embodiments of the invention enable a very high-resolution adjustment of an individual mechanical resonator frequency or the relative frequencies of a multi-resonance device such as in a vibratory microgyroscope. In addition, embodiments of the invention can be performed on assembled devices and in a vacuum chamber using routine electronic or optical stimuli and sensing methods allowing continuous in-situ re-adjustment, without need to break the chamber vacuum. A key feature of present invention lies in the in-situ application where FIB parameters can be adjusted in response to real time monitoring of the gyro performance, e.g. frequency splits; the gyro operates while the tuning adjustments (FIB micromachining) are taking place.
Embodiments of the invention are particularly useful in navigation applications that use precisely tuned mechanical resonators, e.g. vibratory gyroscopes.
Employing the present invention, new microgyros having a drift performance of 0.1 to 0.01 deg/hr can be made. Furthermore, the performance of existing microgyros can be improved to a threshold level required for space applications, e.g. approximately 0.1 deg/hr. For example, the cloverleaf microgyro of U.S. Pat. No. 5,894,091, which is incorporated by reference herein, or various tuning fork rate sensors can benefit from the present invention.
Embodiments of the invention provide an affordable tuned vibratory gyroscope with navigation grade performance by means of a precision isolated symmetric planar resonator of optimum scale that can be fabricated with silicon photolithography from commercial double-side polished silicon wafers with low total thickness variation. Previous navigation grade vibratory gyroscopes with isolated resonators have relied on conventional lathes or milling machines yielding slow and expensive 3D precision machining and assembly, e.g. quartz hemispheric resonator gyroscopes, or employed non-isolated resonators mounted on low-frequency isolators to gain a degree of isolation at the expense of increasing seismic suspension mass and increased deflections due to gravity loads. Asymmetric tuning fork vibratory gyroscopes provide isolation about the drive axis only and are subject to external disturbance about the output sense axis. The cloverleaf microgyroscope of U.S. Pat. No. 5,894,091 as previously mentioned is subject to external disturbances about its drive and output axes.